Idler pulleys are generally formed of metal, stamped or cast, but may also be molded of plastic. It is known to form an idler pulley of two molded plastic pieces, and then join them together afterwards by various means. The U.S. Pat. No. 4,193,310, to Boyer et al, shows a two piece plastic idler pulley in which two separately molded sheaves are held together by rivets. Clearly, the strength of such a unit is limited to the strength of the separate fastening members, and a pulley molded of a single piece would have inherently greater strength. However, such a pulley, as may be seen in U.S. Pat. No. 4,177,685, to Delancy. FIG. 1, would, of necessity, have some parting line left in the V-belt groove by a movable mold member. In addition, if it were desired to insert a central bearing or other separate member in the hub, it would have to be separately placed into and held in the mold.